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Soil-Borne Diseases

Soil borne diseases such as seed rot, root rot and seedling blights often are composed of a complex of root pathogens. This disease complex is caused by a number of soil-borne fungi: Pythium species, Fusarium solani f.sp. pisi, F. oxysporum f.sp. pisi and Rhizoctonia solani. They can attack pea plants – either individually or collectively – any time between germination and maturity. Together they cause a complex of seed rot, damping off, seedling blight, root and foot rot diseases.

Since these root rot pathogens are rarely found individually in a field, control measures should aim to control them all. Possible prevention/control measures include:

  • a four to five-year rotation out of pea or other susceptible hosts (alfalfa and clovers)
  • use seed with high vigour practise good soil fertility, liming of acid soils and ripping to reduce soil compaction
  • avoid herbicides such as MCPA, MCPB or a mix of MCPA and MCPB – these herbicides cause plant stress and make root rot more severe
  • Thiram 75WP® (thiram), Captan FL® (captan) and Vitaflow 280 are registered for control of seed decay, damping off, root rot and seedling blight
  • Apron FL® (metylaxyl) is registered for control of seed rot and seedling blight specific to Pythium species and can be mixed with Thiram 75WP® to give a broader control spectrum

Root Rot in Peas and Lentils in Western Canada

Fusarium Root Rot

  • caused by Fusarium solani (Mart.) Sacc. f. sp. pisi (Jones) Snyd. and Hans.
  • yield losses of 26 per cent to 57 per cent have been reported in both dryland and irrigated areas
  • fusarium root rot is most prevalent under poor crop rotations, high soil temperatures (22˚ to 30˚ C), moist soils, acidic soils (pH 5.1 – 6.2) and low fertility
  • soil compaction by farm machinery also increases both incidence and severity

Symptoms

  • reddish brown streaks initially develop on the primary and secondary roots
  • these eventually join together to form a dark reddish-brown colour on the primary root up to the soil line
  • greying, yellowing, death of lower foliage and stunting can occur if infection is severe
  • foliar symptoms often appear following warm temperatures and heavy rainfall

Fusarium Wilt

  • caused by Fusarium oxysporum Schl. f. sp. pisi (Van Hall) Snyd. & Hans.
  • of the disease’s many races, races 5 and 6 are most important in Western Canada
  • pathogen can be seed or soil-borne and can infect soils for 10 or more years
  • races 5 and 6 thrive under soil temperatures of 20˚ to 21˚ C

Symptoms

  • race 5 and 6 symptoms are most often found in small, circular patches in fields
  • in roots cut longitudinally, a yellowish, orange colour is seen in the vascular tissue, which can extend into the basal area of the stem
  • downward curling of the leaves and stipules
  • thickening of the basal internode and brittle leaves and stem
  • yellowing of the leaves progressing from the base of the plant to the top of the foliage (plants often die due to loss of the root system)

Pythium Root Rot

  • caused by a number of different Pythium species. In Alberta, P. irregulare and P. ultimum are the most common
  • infection very dependent on seed quality – cracked seeds leak more nutrients and are more likely to attract Pythium and be infected than intact seed
  • more prevalent in cool, wet, poorly drained soils

Symptoms

  • seeds are rotted – when removed from the soil, they emerge with a layer of soil around them (this is full of whitish, threadlike fungal hyphae)
  • emerging plant and roots, if produced, may be soft and watery; cotyledons may or may not be rotted
  • germinating seeds are only susceptible for 48 to 72 hours – once the root emerges, the seed is no longer susceptible to infection (new developing tissue, however, remains susceptible)
  • infection can occur at the tip of feeder roots where young tissue can be destroyed – this can lead to root pruning and/or reduction in length
  • depending on the severity of the infection, seedlings may become stunted and chlorotic and collapse as the root base decays and turns tan to light brown
  • infected plants tend to lack vigour and often yield poorly

Rhizoctonia Root Rot

  • caused by Rhizoctonia solani Kuehn.
  • seedlings generally less susceptible as they get older
  • high soil temperatures (24˚ to 30˚ C) are known to cause higher rates of infection

Symptoms

  • infection occurs (and symptoms appear) close to the soil surface
  • symptoms on seedlings first appear as water-soaked lesions that eventually turn reddish brown
  • plant’s growing point may die as it emerges from the ground – this can lead to multiple shoots emerging and then dying
  • on older plants, symptoms appear as reddish-brown sunken lesions on the lower stem, which can cause girdling and often leads to stunted plants

Foliar Diseases

Ascochyta/Mycosphaerella Blight

  • this disease complex is caused by three fungi: Mycosphaerella pinodes (Berk. & Blox.) (the perfect stage of Ascochyta pinodes), which causes blight; Ascochyta pisi Lib., which causes leaf and pod spot and Phoma medicaginis var. pinodella (Jones), which causes foot rot
  • M. pinodes is the most common on field pea in Western Canada – average yield losses run to 10 per cent, but losses of up to 80 per cent have been reported
  • the three species often occur together and can be difficult to tell apart
  • for most pea varieties, expect a 5 to 6 per cent yield reduction for every 10 per cent of the stem area affected
  • these pathogens can be seed, stubble or soil-borne – seed-borne infection by M. pinodes is considered to be the primary source of infection in virgin pea fields
  • P. medicaginis var. pinodella and M. pinodes are both quite persistent, surviving on pea straw fragments and in the soil
  • when moisture and temperature conditions are favourable, residual pycnidia from M. pinodes mature, new pycnidia develop and pycnidiospores are released – these can infect healthy plants by rain splash (ascospores are also produced and can be carried by wind for a kilometre or more)
  • A. pisi, on the other hand, competes very poorly with other microflora in the soil and overwinters very poorly – the main source of infection is from seed-borne spores

Symptoms

  • A. pisi lesions are partially sunken, tan colored and surrounded by a well-defined dark brown margin:
    • circular lesions are found on pods and leaves while lesions on stems are more elongated
    • numerous pycnidia are usually found in the lesions
    • lesions of A. pisi are rarely found on any plant parts below the soil line
  • if lesions on leaves, stems and pods are widespread and severe, M. pinodes is the cause
  • if lesions are severe on roots, P. medicaginis var. pinodella is the cause
  • M. pinodes produces small, brown to purplish, irregular flecks without definite margins, initially appearing on pods, leaves, stems and the cotyledonary area – these lesions enlarge if weather conditions are favorable (15˚ to 18˚ C and high humidity)
  • dark-brown to black specks (pycnidia) are eventually produced and form a distinct concentric tan and brown ring pattern as the lesions enlarge – this is often more pronounced on leaves and pods than on other plant parts
  • lesions developing on stems tend to form long, wide purple to bluish-black streaks that eventually coalesce and may completely girdle the stems, pod attachments or tendrils – these streaks are more common near the nodes and on the lower portion of the stem
  • early pod infection can lead to seed infection, which may show no visible symptoms if infection is light, to varying degrees of shrinkage and discolouration if severe
  • under drier conditions, the concentric ring pattern of the symptoms is less pronounced and may show up only as a uniform yellowing of lower leaves – if the blossom becomes infested, girdling of the sepal often occurs, leading to pod drop or distortion

Prevention & Control

  • have pea seed examined for Ascochyta presence by an accredited seed lab
  • use only pathogen-free seed – if pathogen-free seed cannot be found, use seed with as low a level of Ascochyta as possible, and have the seed treated
  • seed treatment with Thiram 75WP®, although not registered as a control, can give excellent control of seed-borne M. pinodes on pea
  • avoid seeding next to any previous year’s pea fields since spores can spread by wind
  • Bravo 500® (chlorothalonil) has recently been registered for use on field pea in Alberta to prevent infection by Mycosphaerella blight – apply in the early bloom stage and repeat up to three times at 10- to 14-day intervals if conditions favor disease development (warm and humid)
  • bury all crop residue, to prevent the fungus being spread by wind and rain
  • have a four to five-year rotation between pea crops

Sclerotinia Rot

  • caused by Sclerotinia sclerotiorum (Lib.) de Bary (often called white mold or pod rot)
  • prevalent when pea and canola are grown in the same rotation
  • dense canopies and high humidity favour sclerotinia outbreaks
  • ascospores cannot infect pea plants directly because to germinate, they need a source of dying tissue (such as stamens or flower petals) – these are then colonized by threadlike running hyphae, especially under moist conditions
  • ascopores can be released over long distances (up to several kilometres) and long periods of time
  • the critical infection period is during the flowering stage
  • when pea seed is harvested, sclerotia are either harvested with the seed or fall to the ground with the vines – only sclerotia in the top 6 cm of soil can produce apothecia, sclerotia buried deeper remain dormant and can survive in the soil for five to seven years, until conditions for apothecia formation are favourable

Symptoms

  • first sign is the appearance of a light brown discolouration on the stem, leaves or pods
  • with pod rot, this occurs at the base of the style with a small water-soaked lesion appearing on green tissue at the end of the pod
  • with stem rot, the symptoms are more common in the node area
  • whitish, threadlike mats develop over the affected areas and, over time, cause the tissue underneath to turn soft and decay – black, hard bodies (sclerotia) form in the mat and within the stem and pods
  • affected plants often appear wilted and ripen prematurely due to rotting stems – lodging is common in affected areas
  • when humidity is high and moisture is abundant (such as within a thick canopy), tan, fleshy, golf-tee shaped apothecia emerge on the sclerotia – numerous ascospores develop within the apothecia and are released into the air to infect nearby healthy plants
  • lesions appear four to five days after infection, and within eight days, dark to greyish-brown lesions ranging from 1 to 5 mm can be seen on the infected tissue

Prevention & Control

  • four to five-year rotation out of susceptible crops (canola, mustard, sunflower, dry bean)
  • bury sclerotia bodies deep
  • planting semi-leafless pea varieties to allow greater air flow through the canopy
  • no fungicides are registered for use on field pea, but you can control this disease in canola – Benlate 50WP® (benomyl) and Rovral® (iprodione) are registered

Powdery Mildew

  • caused by Blumeria pisi f.sp. pisi
  • borne on seed or stubble
  • seldom a problem until late in the season when environmental conditions are favourable
  • summer rains damage the fruiting bodies, causing them to burst instead of germinate – in late-summer or early-fall, when dews occur without rain, the disease can spread very quickly
  • severe infections reduce plant weight, seed weight, number of seeds per pod, pods per plant and plant height

Symptoms

  • first appear as off-coloured spots on the upper surface of the lowest and oldest leaves
  • spreads rapidly to cover entire surface of leaves, stems and pods with a fine, powdery, bluish-white mildewy growth – underneath the mildew, the tissue often turns purplish
  • as plants age, tiny pinhead size fruiting bodies (cleistothecia) develop – these are brownish at first, eventually turning black at maturity
  • infection can also wither foliage – if infection occurs on the pods, penetration of the pod may occur and cause the seed to turn greyish-brown (severe infections can cause hollow seeds)

Prevention & Control

  • seed early and use earlier maturing varieties
  • use crop rotation practices
  • bury the crop residue
  • Kumulus DF®, an 80 per cent sulphur product is registered for prevention of powdery mildew on pea in Western Canada
  • powdery mildew resistance is being incorporated into all new pea varieties and should be commercially available by the year 2004

Downy Mildew

  • caused by Peronospora viciae, this disease likes cool, wet growing conditions and usually occurs on early-seeded crops
  • pathogen can be seed or soil-borne and can survive in the soil for up to 15 years
  • seed-borne infection often causes failed germination

Symptoms

  • can be expressed systemically, locally (on leaves, tendrils and flowers) or as pod infections
  • systemic infections: produce the most severe effects, causing stunting, distortion and the proliferation of white, cottony growth on the plant; profuse sporulation of the fungus occurs on the surface of the plant, and plants often die before flowering; if pods are produced, they are flattened, yellow, distorted and rarely set seed
  • local infections: appear as fluffy, white to bluish, cottony patches on the undersides of leaflets, while the upper sides turn yellow and die
  • pod infection: first appears as blotches that become brown with green areas inside them, which cause small, brown, sunken spots on the seeds and if severe enough, can lead to seed abortion – a cottony growth can occur inside the pod and may contain numerous oospores
  • infections on older plants may only appear as spotting on the leaves

Prevention & Control

  • use four- to five-year crop rotations
  • practice deep burial of crop residue
  • use of disease-free seed
  • Apron FL®(metalaxyl) is not registered for control of downy mildew, but does provide some protection against soil and seed-borne infections

Bacterial Diseases

Bacterial Blight

  • caused by a bacteria, Pseudomonas syringae pv. pisi (Psp)
  • overwinters mainly as a seed-borne pathogen – seed can remain infected for up to three years
  • rate of infection increases with soil moisture
  • injury from hail, frost, wind, animals or machinery predisposes plants to infection
  • secondary spread occurs by rain splash, wind, machinery, animals and irrigation

Symptoms

  • shiny, water-soaked lesions initially appear near nodes and stipules – these spread to the stems, pod stems and tendrils and become darker in colour
  • the underside of leaves and stipules develop water-soaked spots that appear dark green to brown on the upper surface – with age, spots develop an angular shape with dark margins and a light coloured, papery look in the centre 
  • lesions appear translucent when held up to the light
  • lesions on pods are also water-soaked at first, turning darker as they mature and becoming sunken – pods are often infected through the suture
  • infected seeds may or may not show injury symptoms – watery, dark spots sometimes appear on the seed but usually no visible symptoms appear

Prevention & Control

  • since the pathogen is mostly seed-borne, use disease-free seed
  • seed production should be limited to more arid areas and irrigation should be avoided

Pink Seed

  • caused by Erwinia rhapontici, this disease was first found on pea in southern Alberta in 1988
  • bacteria enter pea pods through wounds caused by insects or mechanical injury and infect the seed inside the pods (seed quality is affected more than yield)
  • more common in irrigated than in dryland pea fields because high humidity and mechanical injury from irrigation allow bacteria to penetrate through wounds to infect the pea plants

Symptoms

  • tan to dark-brown lesions develop on pods only
  • seed turns pink and is often shrivelled

Prevention & Control

  • use disease-free seed

Minor Diseases

Aphanomyces Root Rot

  • caused by Aphonomyces eutiches f.sp. pisi

Symptoms

  • first appear on roots as straw-coloured lesions that gradually spread – this causes a soft, watery, distinctive rot close to the soil line
  • also leads to yellowing from the bottom up and stunting of the plant as it is unable to extract nutrients and moisture from the soil
  • survives in soil for up to 10 years

Prevention & Control

  • a four to five-year crop rotation will minimize damage from this pathogen

Thielaviopsis Root Rot

caused by Thielaviopsis basicola Berk & Br. (also known as black root)

Symptoms

  • often occurs with Fusarium solani – together they cause necrosis and blackening of the entire root system (occasionally, small black lesions also occur on the lower leaves of infected plants)
  • high soil temperatures (28˚ C) favour this disease

Prevention & Control

  • none known

Septoria Blotch

  • caused by Septoria pisi West
  • either seed or soil-borne, although seed transmission is not considered important
  • temperatures of 21˚ to 27˚ C (with high humidity) favour development of this pathogen

Symptoms

  • blotchy lesions with no distinct margins – these develop mainly on the lower, older leaves, pods and stems of the pea plant
  • early blotches are yellowish-green, eventually darkening and enlarging as they mature (irregular in size and shape)
  • entire leaves or stipules may be covered by blotches as they coalesce
  • black specks (pycnidia) develop profusely over the surface of the affected areas

Prevention & Control

  • a four to five-year crop rotation

Grey Mold

  • caused by Botrytis cinerea Pers.exFr., grey mold can occur as a rot on pods, stems and leaves on pea plants or on fresh seed and pods in transit or storage
  • infection occurs quickly under conditions of 16˚ to 21˚ C and 100 per cent humidity

Symptoms

  • fuzzy, grey lesions are the first symptoms to appear on lower areas of the stem and spread until the entire lower foliage turns a fuzzy, grey colour – this appearance is caused by the abundance of conidia produced on leaves and stems
  • as the disease progresses, leaves become shrivelled and dry, and defoliation of bottom leaves occurs
  • pod infection causes the most damage, and clinging blossoms provide a humid environment from which the tip of the young pod can be infected – small, oval, water-soaked lesions develop and spread up the pod (these lesions are tan at first but turn greyish with age and often develop sclerotia in the form of small black specks)

Prevention & Control

  • potassium fertilizer in potassium deficient soils reduces the severity of grey mold

Anthracnose

  • caused by Colletotrichum pisi Pat
  • seed or soil-borne
  • warm temperatures, high humidity and recurrent rains all favour this disease

Symptoms

  • pod lesions are circular and sunken with reddish-brown margins and reddish centres
  • close examination of pod lesions often reveals fruiting bodies (acervuli) with orange-pink spores
  • leaf and stipule lesions are oval, with brown margins and greyish centres
  • stem lesions are elongated – they appear a copperish colour when moist and greyish when dry
  • severely diseased plants have an overall reddish-brown appearance

Prevention and Control

  • use disease-free seed
  • use four to five-year crop rotations out of pea or other hosts

Alternaria Blight

  • Alternaria alternata (Fr.) Keissl. is the causal organism of this disease
  • favoured by prolonged high humidity (more than three days) and warm temperatures (16˚ to 24˚ C)

Symptoms

  • foliar lesions are oval with, a concentric ring pattern inside – they are tannish-brown in the centre and fade out towards the margins
  • up to half or more of the leaves and stipules can be covered in lesions when an outbreak occurs
  • pod lesions are smaller, raised and brownish in appearance – some slight stunting of pea plants can also occur

Prevention & Control

  • crop rotation with non-host crops is the best means of prevention

Cladosporium Blight

  • caused by Cladosporium pisicolum – often referred to as ‘scab’
  • seed or soil-borne
  • occurs under conditions of high humidity and temperatures of 16˚ to 21˚ C

Symptoms

  • first appear as inward curling as leaves unfold
  • leaf lesions can be irregular or circular in shape
  • grey at first, eventually turning a tan to brown colour (margins on lesions are thin and dark brown)
  • shredding of leaves can occur as necrotic areas fall out or tear from wind
  • brownish-black lesions develop on most other plant parts except for the pods (cankers often form as these lesions age and crack)
  • infected pods are often pockmarked and feel bumpy – dark, irregular pimples occur randomly on pods (seed infection can occur if pods are penetrated)
  • severely infected seeds develop black, circular spots with well-defined borders – a light infection may show up as light, scattered black spots on the seed

Prevention & Control

  • use disease-free seed
  • use four to five-year crop rotations

Black Leaf

  • caused by Fusicladium pisicola

Symptoms

  • small, white spots on the underside of leaves are the first symptoms, and these spots penetrate the leaf as they enlarge, forming a concentric ring pattern progressing from tan in the centre to grey to black at the edges – spots eventually darken, as specks (conidia) form on the underside of the leaf
  • black, elongated lesions then streak the leaves – this starts with bottom leaves and progresses upwards (blackening is due to closely-packed conidia on the leaf surface)
  • bottom leaves eventually turn completely black, dry out and become shredded in appearance

Prevention & Control

  • use resistant varieties

Brown Spot

  • caused by Pseudomonas syringae pv. syringae
  • can be seed or soil-borne – does not survive for long on the seed but survives well in the soil
  • 24˚ C and higher temperatures and high humidity favour brown spot

Symptoms

  • initially appear as small, water-soaked lesions on leaves, petioles and stems – these eventually turn a tan colour and appear burnt
  • stem lesions are sunken and tend to elongate upwards – they often cause stems, petioles and growing points to become distorted
  • infected leaves eventually dry up and fall off

Prevention & Control

  • use of disease-free seed
  • four to five-year rotation out of susceptible crops (e.g. dry bean)

Viruses

More than 50 viruses are known to infect pea worldwide. Of these, only a few occur in Canada. Damage is rarely serious in Western Canada because of severe winters and a relatively short growing season.

Economically important viruses in North America are pea seed-borne mosaic virus, pea enation mosaic virus, bean (pea) leaf roll virus, pea streak virus and red clover mosaic virus causing pea stunt. In the field, the pea aphid is the main vector of viruses from infected to healthy plants although other aphids are known to transmit viruses as well.

Symptoms

  • may include downward curling of leaflets on seedlings, leaf mosaic, rosetting, shortening of internodes, chlorotic flecks or translucent lesions, upward curling of leaves, pod distortion and stunting
  • tissue proliferation may occur along leaf veins and on pods
  • pod set may be affected in some cultivars or distorted pods may develop with seeds that have split seed coats
  • seed size and quality is often reduced
  • delayed maturity may also occur
  • some varieties may be infected and show no visible symptoms, but yield is reduced

Prevention & Control

  • use of resistant varieties and virus-free seed
  • control of aphid populations has not proven a reliable control method